In recent years, a rapidly increasing number of electronic devices are becoming portable and cordless. The driving power sources used for such devices are small-sized consumer-use secondary batteries which are small in size and light in weight and have a high energy density. Further, for uses as driving power sources for power storage equipment and electric vehicles, large-sized secondary batteries are being developed. These secondary batteries are required to have characteristics such as high output characteristics, long-term durability, and safety. Under these circumstances, development is actively carried out for non-aqueous electrolyte secondary batteries with high voltage and high energy density.
However, if non-aqueous electrolyte secondary batteries represented by lithium ion secondary batteries are misused, for example, externally short-circuited or overcharged, the battery temperature might increase abruptly. In view of this, a safety mechanism such as a positive temperature coefficient (PTC) element or a safety unit (SU) circuit is incorporated so that the safety of the battery can be ensured. In addition, a function of shutting down current flow in response to an increase in the battery temperature (i.e., a shutdown function) is imparted to a separator interposed between the electrodes in the battery.
A commonly used separator for a non-aqueous electrolyte secondary battery is a polyolefin porous film. When the battery temperature increases to a certain temperature, the polyolefin porous film softens to close the pores therein. As a result, the ion conductivity between the electrodes is lost, and the battery reaction is stopped. Such function is known as a shutdown function. Despite this, if the battery temperature increases even after the shutdown, the polyolefin might be damaged by melting (meltdown), causing a short circuit between the positive and negative electrodes.
The shutdown for ensuring the safety and the meltdown causing a short circuit are both caused by the softening or melting by heat of a resin constituting the separator. It is very difficult, therefore, to prevent the meltdown effectively, while improving the shutdown function. In order to solve this, for example, a composite film formed by combining a polyolefin porous film with a highly heat-resistant layer is being developed.
With respect to non-aqueous electrolyte secondary batteries required to have excellent output/input characteristics, they are expected to have an ability to be rapidly charged. In order to ensure a sufficient capacity retention rate in charge/discharge cycles for rapidly charging the battery, it is necessary to improve the movability of lithium ions between the electrodes. However, in order to improve the movability, if the pore diameter of the separator is simply increased, the growth of needle-like metal lithium (dendrites) is facilitated, and an internal short circuit tends to occur. If the dendrites are dissociated, the capacity of the battery is reduced, or the pores in the separator are clogged. Alternatively, if the porosity is simply increased, the tensile strength or the piercing strength of the separator is lowered, or the handleability of the separator is degraded.
In view of the above, various proposals have been made with regard to the structure of the separator.
Patent Literature 1 discloses that a fluorocarbon resin raw material such as polytetrafluoroethylene be stretched at a high temperature of about 300° C., to produce a separator having nodes extending in the film thickness direction. Patent Literature 1 teaches that the nodes extending in the film thickness direction improve the compression strength of the separator and suppress the formation or dissociation of dendrites.
Patent Literature 2 discloses that a non-oriented polyolefin resin layer be laminated on at least one surface of a biaxially-oriented polypropylene porous film. Patent Literature 2 teaches that this can improve the handleability of the film while maintaining its excellent air permeability and high porosity.
Patent Literature 3 discloses a separator comprising a polyolefin resin including a high-molecular-weight polyethylene having an average molecular weight of 500,000 or more and a polyethylene having an average molecular weight of less than 500,000. Patent Literature 3 teaches that by using such a polyolefin resin, the shutdown temperature can be lowered while the piercing strength is maintained, and the shrinkage percentage when shutdown occurs can be reduced.